LONDON, Sept 14 (Reuters) - The United States must consider the full threat it says Iran poses to the Middle East when formulating its new policy toward Tehran, U.S. Secretary of State Rex Tillerson said on Thursday, adding that Iran had breached the spirit of a 2015 nuclear deal.

Tillerson, made the comments during a visit to Britain to see Prime Minister Theresa May and foreign minister Boris Johnson. The trip was billed as focusing on the relief effort after Hurricane Irma, how to respond to North Korea’s nuclear test, and resolving the political deadlock in Libya.

But he was outspoken in his criticism of Iran when asked whether he believed it was meeting the obligations of a 2015 international nuclear agreement designed to curb an Iranian nuclear programme in return for lifting most Western sanctions.

Tillerson cited the preface of the nuclear deal, which calls on Iran to contribute positively to regional security.

“In our view, Iran is clearly in default of these expectations ... through their actions to prop up the Assad regime (in Syria), to engage in malicious activities in the region, including cyber activities, aggressively developing ballistic missiles,” he told a news conference.

“We have to consider the totality of Iran’s activities and not let our view be defined solely by the nuclear agreement.”

He was speaking before the United States announced it had imposed sanctions on seven Iranian individuals and two entities, alleging involvement in either malicious cyber activities or enabling Tehran’s nuclear programme.

U.S. President Donald Trump has previously expressed doubts about the nuclear deal, and in April his administration said it would review whether the lifting of sanctions against Iran was in the United States’ national security interest.

Trump is weighing a strategy that could allow more aggressive U.S. responses to Iran’s forces, its Shi’ite Muslim proxies in Iraq and Syria, and its support for militant groups.

NORTH KOREA

Earlier on Thursday, a North Korean state agency threatened to reduce the United States to “ashes and darkness” for supporting a U.N. Security Council resolution and sanctions over its latest nuclear test.

Tillerson said he was hopeful that China would decide to use the “powerful tool” of oil supplies to put pressure on Pyongyang, but conceded that it would be difficult to agree an oil embargo through the United Nations Security Council.

North Korea carried out its sixth and largest nuclear test earlier this month.

“I am hopeful that China, as a great country, a world power, will decide on their own, will take it upon themselves to use that very powerful tool of oil supply to persuade North Korea to reconsider its current path towards weapons development, reconsider its approach to dialogue and negotiations in the future,” he said.

Commenting on violence in Myanmar against the minority Rohingya population, Tillerson said the country faced a defining moment.

“I think it is important that the global community speak out in support of what we all know the expectation is for the treatment of people regardless of their ethnicity,” he added. “This violence must stop, this persecution must stop.”

He said he understood that Myanmar’s Aung San Suu Kyi, a Nobel prize laureate and de facto head of the government in Myanmar, was in a power-sharing agreement with the military and it was a “complex situation” in which she found herself. (Additional reporting by Elizabeth Piper and Kylie MacLellan; Editing by Alison Williams)